The present invention is directed to processes for the synthesis of polyol fatty acid polyesters by transesterification of a polyol and using inert gas to remove methanol. More specifically, the present invention is directed to an improved inert gas sparging process employing intense agitation of the fluid in the column to improve mass transfer.
Processes for the synthesis of polyol fatty acid polyesters by the transesterification of a polyol are well known in the art. For example, the Rizzi et al. U.S. Pat. No. 3,963,699 discloses a solvent-free transesterification process comprising two main steps, each of which is conducted in a batch reactor. In the first step, a mixture of polyol, a fatty acid lower alkyl ester, an alkali metal fatty acid soap, and a basic catalyst are heated to form a homogenous melt of partially esterified polyol and unreacted starting materials. In a second step, excess fatty acid lower alkyl esters are added to the reaction product of the first step to form the polyol fatty acid polyester. Rizzi et al. further disclose that a lower alcohol is formed as by-product of the reaction and, in order to promote the reaction, the alcohol by-product is preferably removed. Many removal techniques are acknowledged by Rizzi et al. as being known in the art; Rizzi et al. indicate that vacuum removal, both with and without an inert gas, has been found to promote the reaction, and that simple distillation under atmospheric pressure may also be sufficient.
The Volpenhein U.S. Pat. Nos. 4,517,360 and 4,518,772 disclose further solvent-free transesterification processes for producing higher polyol fatty acid polyesters. In U.S. Pat. No. 4,517,360, Volpenhein discloses the use of potassium carbonate, sodium carbonate or barium carbonate as a catalyst and the use of a fatty acid methyl 2-methoxy ethyl or benzyl ester. In U.S. Pat. No. 4,518,772, Volpenhein discloses the use of preferred molar ratios of soap to polyol of from about 0.6:1 to about 1:1 in the first step of the two step process. Volpenhein also employs a batch reaction process and discloses the advantage of removing lower alcohol by-product to promote the transesterification reaction.
The Buter U.S. Pat. No. 5,043,438 discloses a process for the synthesis of polyol fatty acid esters by reacting a polyol and a fatty acid lower alkyl ester under substantially solvent-free conditions. Buter discloses that the process employs a pre-reactor in which the reaction mixture is in steady state with mass-balanced in-going reactant streams and out-going product streams having a polyol conversion of 1% or more, and a nonagitated column main reactor, which in the examples was a three-tray column reactor with counter-current stripping gas, i.e. nitrogen.
U.S. Pat. No. 5,767,257 issued to Schafermeyer et al. discloses a method for producing polyol fatty acid polyesters using atmospheric or superatmospheric pressure, and is incorporated herein by reference. U.S. Pat. No. 5,767,257 teaches that an inert gas can be sparged through a reactor with sufficient mass transfer surface area contact between the inert gas and reactor liquid to transfer lower alkyl alcohol byproducts from the a liquid mixture of the reactants.
Other U.S. Patents related to processes for making polyol polyesters include U.S. Pat. No. 5,231,199 to Willemse, U.S. Pat. No. 5,158,796 to Bernhardt, and U.S. Pat. No. 5,518,772 to Volpenhein.
U.S. Pat. No. 5,945,529, titled xe2x80x9cSynthesis of Polyol Fatty Acid Polyesters Using Column with Inert Gas Strippingxe2x80x9d to Corrigan et al. and filed on Jul. 19, 1996 discusses the use of inert gas stripping in the synthesis of sucrose polyesters, and is incorporated herein by reference.
Robert E. Treybal, in xe2x80x9cMass-Transfer Operationsxe2x80x9d Third Edition, 1980 discusses at pages 139-142 discusses sparged vessels, and is incorporated herein by reference.
Removal of lower alkyl alcohol (e.g. methanol) in polyol polyester synthesis is generally desirable to achieve higher degrees of esterification of the polyol. For instance, the equilibrium equation for the synthesis of sucrose octaester from sucrose heptaester and fatty acid methyl ester requires that methanol be removed in order to produce significant quantities of sucrose octaester.
The problems associated with removing lower alkyl alcohol such as methanol is compounded when reactions are to take place on a large scale suitable for commercial application. A vacuum source can be used help extract the methanol, but it is desirable to eliminate the need for vacuum in commercial scale polyol polyester synthesis. Additionally, the static pressure of the liquid reactants in a large scale column reactor tends to compress gas bubbles. The resulting reduction in the surface area of the bubbles results in reduced mass transfer of methanol from the liquid mixture to the stripping gas.
Longer residence times of reactants in the column reactor would generally be expected to increase the transfer of methanol from the liquid mixture of reactants and reaction by-products. However, longer residence times in commercial scale reactors are undesirable from a cost standpoint.
Polyol fatty acid polyesters are increasingly being employed in various applications. Particularly, there has been a significant increase in the use of polyol fatty acid polyesters as low-calorie fats in many food products. Accordingly, the demand for polyol fatty acid polyesters suitable for human consumption is rapidly increasing. As a result, processes for more efficient and economical synthesis of polyol fatty acid polyesters are necessary and desirable. Accordingly, scientists and engineers seek improved methods for removing lower alkyl alcohol from liquid mixtures in the synthesis of polyol polyesters.
It has now surprisingly been found that inert gas can be employed in commercial scale polyol polyester reaction mixtures to provide the formation of turbulent micro-eddies that significantly enhance mass transfer with the reacting phase. In particular, the gas induced formation of turbulence has been found to be effective in promoting the transfer of methanol from the liquid reaction mixture to the inert gas even under atmospheric and superatmospheric reaction pressures. Moreover, the removal of methanol can be accomplished without long reactor residence times.
According to one aspect of the present invention, a process for the synthesis of polyol fatty acid polyester, such as sucrose octaester, comprises reacting polyol with fatty acid lower alkyl ester to form a liquid mixture comprising polyol polyester, fatty acid lower alkyl ester, and lower alkyl alcohol by-product, while sparging inert gas through the liquid mixture. The inert gas is directed through the liquid mixture at a bulk gas velocity sufficient to provide gas induced turbulent flow of the inert gas through the liquid mixture, wherein the gas induced turbulent flow enhances transfer of the lower alkyl alcohol from the liquid mixture. The reaction of the polyol with fatty acid lower alkyl ester can take place at a reaction pressure of between about 0 and about 15 psig.
The reaction can take place in a multistage column reactor having a column ratio of at least 0.4/hour, preferably at least 0.5/hour, and more preferably at least 0.6/hour. The column ratio is defined as the gas flow rate through the column divided by the mass of reacting liquid present in the column, or:
(Inert Gas Flow Through Column (pounds/hour))
(Mass of Reacting Liquid in the Column (pounds))
The mass of liquid in the column can generally be calculated as the product of the mass flow rate of the reacting liquid in the column multiplied by the residence time of the reacting liquid in the column.
In one embodiment, the present invention provides a two step process for the synthesis of polyol fatty acid polyester. The process comprises a first step of reacting polyol, such as sucrose, with a first portion of fatty acid lower alkyl ester to provide a first step reaction product wherein the polyol is at least about 40% esterified. In a second stage of the process, the first step reaction product and a second portion of fatty acid lower alkyl ester are reacted in a multi-stage column reactor to form a liquid mixture comprising polyol polyester, fatty acid lower alkyl ester, and lower alkyl alcohol by-product. The liquid mixture passes in a first direction through the column reactor while an inert gas, such as nitrogen, is directed in a second direction counter current to the flow of the liquid mixture. The inert gas flow rate is selected relative to the mass of the liquid mixture in the column to provide a column ratio of at least about 0.4/hour, and preferably greater than about 0.5/hour.
In a preferred embodiment, the polyol polyester in the column reaction product is at least about 95% and preferably at least about 97% esterified. The polyol polyester can have a degree of conversion of greater than about 95%, and at least about 60 weight percent, more preferably at least about 70%, and still more preferably at least about 75% of the polyol polyester in the reaction product can be fully esterified polyol polyester (e.g. sucrose octaester when the polyol is sucrose). The column reaction can be conducted with a liquid flow rate of at least about 1800 pounds per hour, preferably at least about 10,000 pounds per hour, wherein the residence of the liquid mixture in the column reactor is less than about 3.0 hours, and more preferably less than about 2.0 hours.